The invention concerns a process for the deoxidation of a metal casting which has oxide and/or oxygen enriched layers at or near the surface of the casting material. This process removes surface oxides and/or oxygen to levels that are nearly equal to that found in the bulk of the casting by using calcium as a deoxidant. Processes to reduce ores or metal oxides to metal frequently require extreme temperatures, as shown in the following: U.S. Pat. No. 2,834,667 to Rostron teaches direct thermal reduction of titanium dioxide by using metallic magnesium at temperatures exceeding 1000.degree. C. U.S. Pat. No. 2,537,068 to Lilliendahl et al. shows the reduction of zirconium oxide or double chloride with calcium at temperatures between 1100.degree. and 1200.degree. C. U.S. Pat. No. 2,653,869 to Gregory et al. teaches the production of vanadium powder from vanadium trioxide mixed with calcium and calcium chloride at temperatures from 900.degree. to 1350.degree. C. U.S. Pat. No. 4,519,837 to Downs discusses a process for reducing metal oxide powders using molten lithium and magnesium or molten lithium and calcium metals at 600.degree. C.
During the investment casting process, molds are made from refractory oxide or silicate slurries which are coated onto wax patterns. These molds are then fired at sufficiently high temperatures to remove the wax pattern and completely dry the mold. These molds are then cast by titanium or a titanium alloy under an inert atmosphere; however, a reaction between the molten titanium or alloy and the oxidic mold occurs, resulting in the formation of a thin layer of titanium oxide (.alpha.-case) at or near the surface of the casting part. These oxygen enriched areas form very hard surface layers of low ductility which cause deterioration of strength and mechanical properties in the casting.
These oxygen enriched layers present at or near the surface of the titanium casting can be removed by grinding or pickling with acid solutions; however, these methods are difficult to control resulting in high metal losses. Other traditional methods for removing these oxide layers, like shot blasting, also suffer from similar limitations.
The use of calcium as a metal deoxidant is well known. Prior art methods require high temperatures and an excess of pure, expensive calcium. U.S. Pat. No. 4,923,531 to Fisher illustrates the use of a mixture of molten sodium and calcium at 950.degree. C. to remove oxygen from thin titanium scraps and powders to very low levels. U.S. Pat. No. 5,022,935 to Fisher discusses the use of calcium metal to remove bulk oxygen from titanium scraps and powders.